Studies of self-assembled metal-organic nanostructures and the MBE growth of graphene

Summerfield, Alex (2016) Studies of self-assembled metal-organic nanostructures and the MBE growth of graphene. PhD thesis, University of Nottingham.

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This thesis discusses the formation of metal-organic and organic structures grown on surfaces using bottom-up self-assembly techniques. Three systems are investigated primarily using scanning probe microscopy techniques.

The growth of metal-organic frameworks (MOFs) on functionalised surfaces is investigated using high resolution atomic force microscopy (AFM). The earliest stages of MOF crystal nucleation are imaged using a layer-by-layer (LBL) growth technique and the ability to track the growth of individual nanocrystallites throughout the LBL process is demonstrated. This LBL method has been suggested as a route to fabricating epitaxially grown, oriented thin-films of MOFs. However, results from these studies indicate that, rather than a uniform crystalline layer, the morphology is that of a preferentially oriented but laterally polycrystalline film and the growth rates of the individual nanocrystallites exceed those expected for a LBL growth mode. This has significant implications for the fabrication of novel devices that incorporate MOFs due to the presence of domain boundaries and defects.

Self-assembled monolayers of light-harvesting porphyrin nanorings are investigated with scanning tunnelling microscopy (STM) and AFM. The nanorings are found to form large supramolecular networks in ambient conditions on graphite and boron nitride surfaces. The size and order of these networks is found to be dependent on the number of porphyrin macrocycles that make up each ring. In addition, simulations of isolated nanorings are also performed using Monte Carlo methods to model the distortion previously been observed for isolated nanorings on gold surfaces. These are discussed in the context of spectroscopic measurements which suggest that both size dependent and thermally induced distortion affects the lifetime and delocalisation of excited states in these molecules.

Graphene is grown on hexagonal boron nitride surfaces using high-temperature molecular beam epitaxy. Large domains of monolayer graphene are successfully grown and are investigated using AFM and Raman spectroscopy. These domains are found to exhibit hexagonal moiré patterns on the graphene surface which is suggestive of orientational alignment with the underlying boron nitride substrate. Regions with high period and distorted moiré patterns are also observed which suggest that the graphene is under tensile strain which is attributed to the high growth temperatures used. The strain is found to significantly affect the Raman spectrum of graphene and a relationship between the strain and the shifting of Raman spectral peaks is determined. Successful attempts are also made to modify the strain in the graphene monolayer using an AFM tip which is observed to relax when defects are introduced in a controlled manner to the graphene monolayer. These results represent new approaches to the introduction and control of strain in graphene which may be useful for the fabrication of high-performance graphene devices.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Beton, P.H.
Moriarty, P.J.
Keywords: Graphene, Molecular Beam Epitaxy, MBE, Atomic Force Microscopy, AFM, Scanning Tunneling Microscopy, STM, Hexagonal Boron Nitride, Metal Organic Frameworks, MOFs, Raman Spectroscopy
Subjects: Q Science > QD Chemistry > QD450 Physical and theoretical chemistry
Faculties/Schools: UK Campuses > Faculty of Science > School of Physics and Astronomy
Item ID: 33067
Depositing User: Summerfield, Dr Alex
Date Deposited: 20 Jul 2016 12:47
Last Modified: 01 Sep 2016 13:18

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